1. Field of the Invention
The present invention relates to a radio communication system, and more particularly to a power control technique for controlling transmission power of a radio transmitter/receiver using a same frequency.
2. Description of the Related Art
In general, a transmission power of a radio communication system is controlled by controlling a gain of a multistage amplifier in the system. In particular, in a radio communication system sharing the same frequency in both the transmission and reception modes, a battery may be rapidly exhausted because the battery is continuously used for transmitting a signal. Further, in case of using a plurality of radio communication systems at the same time, the radio communication systems may interfere with one another. Therefore, it is necessary to control the transmission power.
An earlier power control circuit includes a transmitter, a receiver, and a central processing unit (CPU). In a transmission mode by the transmitter, a modulation signal output from a modulator is first amplified by a multistage amplifier in an intermediate frequency (IF) stage and transferred to an input of a first mixer via a first filter.
A local oscillator signal output from a voltage controlled oscillator (VCO) is transferred to another input of the first mixer via a first transmission/reception control switch and then the first mixer mixes the modulation signal with the local oscillator signal to generate a radio frequency signal. The radio frequency signal output from the first mixer is filtered by a second filter and supplied to a multistage amplifier in a radio frequency (RF) stage i.e., a drive amplifier and a power amplifier. The gain controlled signal output from the power amplifier is supplied to a coupler and transmitted in the air via second transmission/reception control switch, a third filter, and an antenna. The coupler divides a power of the power amplifier so as to allow a level detector to detect a power level of the gain controlled signal output from the power amplifier. The level detector provides the central processing unit with the level detection result via an analog-to-digital converter. The central processing unit detects the magnitude of the signal output from the level detector and generates control signals to control the outputs of the multistage amplifier, the drive amplifier and the power amplifier so as to control the transmission power. The first and second transmission/reception control switches determine a transmission/reception path under the control of the central processing unit.
In the reception mode, the receiver receives reception data via the antenna and the reception date is supplied to a low noise amplifier by way of a third filter and the second transmission/reception control switch. A signal amplified by the low noise amplifier is filtered by a fourth filter and supplied to an input of a second mixer. The local oscillator signal output from the voltage controlled oscillator is supplied to another input of the second mixer via the first transmission/reception control switch and the second mixer mixes the above-mentioned signals supplied to the inputs thereof to generate a demodulation signal. The demodulation signal is supplied to a demodulator via fifth filter and demodulated into an original signal by the demodulator.
Therefore, when an antenna port of a counterpart's receiver needs a constant power level, a transmission power of the transmitter must be matched with an input level of the counterpart's receiver by using a variable attenuator for varying a gain of the amplifier.
However, since the variable attenuator used for controlling the power includes the multistage amplifier, it is impossible to know a dynamic range of the amplifier, which results into a difficulty in controlling the power. Further, in case of mass-producing the system, it is difficult to finely control the power due to a deviation of the active elements therein.
Further, in the reception mode of operation, since the same frequency is used, the carrier of the transmitter may be reversely supplied to the receiver, which results into a deterioration of the reception sensitivity.
The following patents each disclose features in common with the present invention but do not teach or suggest the specifically recited power control technique for controlling the transmission power of a radio transmitter/receiver using a same frequency as in the present invention: U.S. Pat. No. 5,487,179 to Larsson, entitled Arrangement For Duplex Transmission Having Transmitter Power Control, U.S. Pat. No. 5,551,057 to Mitra, entitled Cellular Mobile Radio System Power Control, U.S. Pat. No. 5,574,993 to Kobayashi et al, entitled Mobile Communication Apparatus And Method, U.S. Pat. No. 5,566,363 to Senda, entitled Transmission Power Control Circuit And Mobile Communication System Incorporating The Circuit, U.S. Pat. No. 4,590,417 to Tanaami et al., entitled Voltage Controlled Diode Attenuator, U.S. Pat. No. 4,553,105 to Sasaki, entitled Multistage Linear Amplifier For A Wide Range Of Input Signal Levels, U.S. Pat. No. 4,999,583 to Washburn et al., entitled Amplifier Drive Controller, U.S. Pat. No. 5,307,512 to Mitzlaff, entitled Power Control Circuitry For Achieving Wide Dynamic Range In A Transmitter, U.S. Pat. No. 5,241,694 to Vaisanen et al., entitled Circuit For Forming Low Power Levels In A Transmitter Of A Radio Telephone, U.S. Pat. No. 5,179,353 to Miyake, entitled Power Control Circuit Used For Both Analog/Digital Dual Mode, U.S. Pat. No. 5,152,004 to Vaisanen et al., entitled Procedure For Forming Low Power Levels In A Radio Telephone Transmitter, U.S. Pat. No. 5,646,578 to Loh et al., entitled Wide Dynamic Range Power Amplifier, U.S. Pat. No. 5,507,016 to Okuhara, entitled Power Control Circuit For A Digital Radio Telephone, U.S. Pat. No. 5,438,683 to Durtler et al., entitled Automatic Level Control Circuit For Dual Mode Analog/Digital Cellular Telephone, U.S. Pat. No. 5,430,411 to Boulic, entitled High-Frequency Amplifier Arrangement, Transceiver Station Comprising Such An Amplifier Arrangement And Mobile Radio System Comprising Such Stations, and U.S. Pat. No. 5,410,729 to Kumagai et al., entitled Signal Transmission Device Having An Automatic Transmission Level Control Circuit Used In A Multi-Carrier Transmission Terminal Equipment And Multi-Carrier Transmission Terminal Equipment Employing The Transmission Device.